A steam generator of said type is known for example from EP 1 288 567 A1.
In a gas and steam turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is utilized for the generation of steam for the steam turbine. The heat is transmitted in a waste-heat steam generator which is connected downstream of the gas turbine and in which a number of heating surfaces for water preheating, for steam generation and for steam superheating are normally arranged. The heating surfaces are connected into the water/steam circuit of the steam turbine. The water/steam circuit conventionally comprises a plurality of, for example three, pressure stages, and each pressure stage may have an evaporator heating surface.
Several alternative implementation concepts, more specifically implementation as a continuous-flow steam generator or implementation as a circulating-flow steam generator, come under consideration for the steam generator connected downstream of the gas turbine as a waste-heat steam generator on the heating gas side. In the case of a continuous-flow steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a once-only pass. In contrast thereto, in the case of a natural or forced circulation steam generator, the circulated water is only partially evaporated during a pass through the evaporator tubes. After the generated steam has been separated, the water that did not evaporate during this process is supplied once again to the same evaporator tubes for further evaporation, the evaporated fraction being replaced by the water newly supplied to the evaporation system.
In contrast to a natural or forced circulation steam generator, a continuous-flow steam generator is not subject to any pressure limitations, so that fresh steam pressures well above the critical pressure of water (Pcri≈221 bar), where there are still only slight density differences between a liquid-like and a steam-like medium, are possible. A high fresh steam pressure is conducive to high thermal efficiency and therefore low CO2 emissions of a fossil-fired power station. Moreover, as compared with a circulating-flow steam generator, a continuous-flow steam generator has a simple type of construction and can therefore be produced with a particularly low outlay. The use of a steam generator designed on the continuous-flow principle as a waste-heat steam generator of a gas and steam turbine plant is therefore especially beneficial for achieving a high overall efficiency of the gas and steam turbine plant along with a simple type of construction.
Particular advantages as regards the outlay in terms of production, but also as regards maintenance work which is required are afforded by a waste-heat steam generator of the horizontal type of construction, in which the heating medium or heating gas, that is to say the exhaust gas from the gas turbine, is routed through the steam generator in an approximately horizontal flow direction. Such a steam generator is known, for example, from EP 0 944 801 B1.
For efficiency or emission reasons, when a steam generator is being started it is desirable to have as short a startup time, as it is known, as possible, that is to say the time until full-load conditions and a thermal equilibrium of the plant components, along with full heating, are reached. A gas turbine requires a comparatively short time for the starting operation or for load change operations. The exhaust gas from a gas turbine can thus reach a high temperature relatively quickly. A short startup time of a gas turbine is desirable because it results in the starting losses occurring during the starting operation and consequently the emissions of the gas turbine being kept low.
If, as is the case in gas and steam turbine plants, a steam turbine is connected downstream of the gas turbine on the heating gas side, the waste heat from the gas turbine is utilized as heating gas in the steam generator of the steam turbine. For technical reasons, in particular due to the thermal inertia of its large masses, the steam turbine requires a longer startup time than the gas turbine and therefore predetermines the lower limit for the necessary startup times. Since the gas turbine therefore “leads” the steam turbine, the waste heat from the gas turbine cannot be absorbed completely by the steam circuit of the steam turbine during the startup operation of the gas and steam turbine plant. For this reason, during the operation of starting the gas and steam turbine plant the major part of the hot steam is usually routed past the steam turbine into the condenser via what is known as a bypass system. During this operation the steam turbine is carefully warmed up by means of a small portion of the steam flow so as to avoid high rates of temperature change which would lead to inadmissible material stresses. Only after this comparatively lengthy warming-up operation of the steam turbine can the latter be acted upon by the full steam volume and make its contribution to the overall power output of the plant. Consequently, only the gas turbine power output is available for a certain period of time during a starting operation.
In order to keep this period of time particularly short or even avoid it completely if at all possible, the steam produced in the steam generator can be cooled before it leaves the steam generator to a temperature level approximating to the material temperatures of the steam turbine plant. This cooling is the more complicated, the further the gas turbine “leads” the steam turbine during startup.
The cooling is typically accomplished using an injection cooling means within the superheater heating surfaces connected downstream of the continuous-flow heating surfaces of the steam generator during the startup operation of the gas and steam turbine plant. However, the temperature of the steam when it emerges from the superheater can be controlled only within certain limits with the aid of injection cooling, with the result that even with measures of this type a certain warming-up time for the steam turbine still generally remains. Moreover, the use of the separate cooling device is technically complex.